p.s. GPL stuff also requires the author, just as any other copyright statement. (public domain, which is not legally recognized by all countries is "without" an author)

If you ask them nicely? Isn't FOSS all about communities and sharing and collaboration etc etc? No?

Additionally to what Ivan said: I think it's also a bit tasteless to ship wallpapers that are part of another projects identity (The same/a very similar wallpaper was the default in GNOME 3.12 IIRC). It could be done, but I don't think it's a good idea.